The invention relates to an internal combustion engine, comprising an engine block with at least one cylinder, with a cylinder head which is fastened to the cylinder, with cooling ribs which are disposed on the outside of the cylinder and the cylinder head in order to discharge the heat produced during the operation of the internal combustion engine, and with an oil-pump to convey the engine oil through the oil lines to the components of the internal combustion engine to be lubricated and to the oil cooler for cooling the engine oil, which oil cooler is provided with a design so as to be integrally included in the cooling ribs of the internal combustion engine.
The use of air-cooled internal combustion engines offers a number of advantages particularly for motorcycles. In addition to the simplicity of the arrangement and the savings in weight, the visual advantages are also worth mentioning, because air-cooled motorcycles are preferred by a large number of riders.
Air-cooling comes with system-inherent disadvantages, however, such as an undesirable limit placed on output and other design parameters by the maximum dischargeable quantity of heat. This disadvantage can be reduced in the known manner by using oil coolers. Such an oil cooler can impair the visual appearance of the motorcycle. A further disadvantage of the concept of air cooling can also not be prevented by using an oil cooler, namely an uneven distribution of heat in the engine. As a result, individual cylinders are cooled better than others depending on the arrangement of the cylinders in the motorcycle and considerable differences in the temperatures occur on the various sides of a cylinder. This leads to increased wear on the material and prevents a substantial optimization of the tolerances. Moreover, it is desirable in modern engine construction that comparably low temperatures prevail in the cylinder head, whereas rather higher temperatures are desired in the cylinder to reduce friction, which higher temperatures should be as even as possible. Such concepts can be realized only with difficulty with air-cooled internal combustion engines of known design.
DE 37 13 849 A describes an internal combustion engine with an oil cooler which is integrated in the cooling ribs. A collecting chamber is provided in the cylinder head, from which the oil emerging from the components to be lubricated is guided either directly to the oilpan or via the oil cooler to the oil pan. In any case, the entire oil passes through the lubrication circulation. The cooling effect is naturally limited in such a design, because, on the one hand, the flow through the oil cooler is limited and, on the other hand, optimal thermal conditions do not always prevail in the oil cooler.
It is the object of the present invention to avoid such disadvantages and to provide an air-cooled internal combustion engine which, without any impairment to the visual appearance, allows optimal cooling in order to allow realizing a high power density.
It is provided in accordance with the invention that two mutually separated oil duct systems are provided in the cylinder head, namely a lubricating oil system and a cooling oil system. Although said systems are flowed through by the same engine oil, the lubricating oil system is used primarily for lubricating movable components, even though a certain cooling effect cannot be excluded. The cooling oil system is used primarily for cooling and is separated within the cylinder head from the lubricating oil system. Assuming a predetermined oil quantity that is required per unit of time for lubrication, the solution in accordance with the invention allows increasing the cooling oil quantity to virtually any desired level should this be required for technical reasons. That is why such a system can be subjected to considerably higher stresses than the solutions according to the state of the art.
The invention further makes use of the finding that the thermal stresses on cooling ribs are not uniform. In the case of a respective advantageous arrangement of the oil cooler, it is therefore possible to considerably increase the total quantity of heat which can be discharged by way of the cooling ribs. The oil cooler can further be provided with a virtually invisible design, so that the visual appearance of a motorcycle with such an engine is not impaired. This is promoted particularly by the fact that the oil cooler is preferably provided with heat exchanger pipes which are designed substantially perpendicular to the plane of the cooling ribs.
A particularly advantageous heat distribution within the engine can be achieved in such a way that the oil cooler is exclusively integrated in the cooling ribs encompassing the cylinder. In this way it is possible to minimize the temperature difference between the cylinder head and the engine block.
It is advantageous when the cooling oil system of the cylinder head is in connection at its downstream side with the oil cooler and when the lubricating oil system of the cylinder head is connected at its downstream side with the return flow. Optimum temperature management can be achieved in this way.
Control is preferably performed in such a way that the oil pump is provided downstream with a relief valve which is permanently connected with the lubricating oil system of the cylinder head and which on exceeding a predetermined oil pressure diverts oil into the cooling oil system of the cylinder head. The relief valve is set in such a way that the supply of the cooling oil system of the cylinder head is cut off in idle operation and at low engine speed.
Only from a predetermined engine speed at which the oil-pump builds up a determinable oil pressure will the cooling oil circulation be activated in order to improve the then inadequate cooling.
It is favorable to ensure that for supplying the cooling oil system the oil line is connected to the return via a return line with a thermostatic valve. The warm-up period can thus be accelerated.
In order to also optimally cool the uppermost region of the cylinder it may be provided that the cooling oil system is partly disposed in the upper zone of the cylinder. The cooling oil can be guided through respective pass-throughs between cylinder head and cylinder.
An even cooling can be achieved in particular in such a way that the oil cooler is disposed in the zones of the cooling ribs which are especially subjected to the cooling air. This concerns primarily the front areas of the cylinder which are flowed against by the relative wind.
In the case of V-type engines with crankshafts disposed transversally to the direction of motion, one frequently encounters the problem that the rear cylinder is at a disadvantage with respect to cooling. Such irregularities can be avoided or at least reduced in that the oil cooler is disposed in the zone of the cooling ribs of only a part of the cylinder. The thermally preferred cylinder(s) is/are chosen for situating the oil cooler.
A particularly effective measure provides that cooling oil lines are provided in the zone of valve seats of the internal combustion engine. It is similarly appropriate to provide cooling oil lines in the zone of the spark plugs.
A preferred connection variant provides that the downstream output of the oil cooler opens into an oil reservoir. This is generally the oilpan of the internal combustion engine when no dry sump lubrication is used.